In the prior art, the electromotive tool, such as an electric drilling device, is driven by electric power. The drilling device is not only used to clamp a drilling head for drilling a hole, but also used to clamp the head of a screw driver for screwing a screw. In the place easily providing with a power, the powered electric drilling device certainly can provide a powerful driving force. But for a place without power, or the receptacle is far away from the working place so that the electric wire is extraneously inconvenient to be connected, a charged electric drilling device can provide a useful selection. However, it is probable that the power of the drilling device is not sufficient or the power is interrupted suddenly, so that it can not used to drive a head or a screw driver, then the drilling device is necessary to be operated manually. However, since in the conventional drilling device, the active gear connected with the driving shaft of a motor is engaged directly with a gear set. Further, a passive gear engaged on the central shaft is driven by the gear set. Therefore, when the drilling device is used manually, the passive gear will become an "active gear", the power will send to the real active gear through the motor driving shaft. Therefore, the drilling head or the screw driver will rotate ineffectively in the original place. Namely, it can not be used as a manual drilling device.
Moreover, conventionally, in the electromotive tool, such as an electric drilling device for clamping a head of a tool, the clamping device installed on the front end thereof is engaged with an output shaft. Further, the clamping device is installed with a plurality of clamping palms and a rotary button coaxial with the output shaft. By rotating the rotary button, the palms are controlled to move to the center to clamp the tool head, or they are radially moved in the outward direction for releasing the tool head. Since the clamping device is installed on the output shaft, as the rotary button is rotated, one hand is necessary to hold a fixing head engaged with the output shaft, while another hand is used to rotate a rotary button. Namely, the operation is performed by two hands. Thus, it is very inconvenient in using.